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340 CHEMICAL ENGINEERING
the use of a certain proportion of creosote can scarcely be avoided. The standard frothing oil is steam-distilled pine oil. Destructively distilled pine oil or other light volatile and miscible oil can often be substituted for it. The steam oil is very expensive. The frothing oils have an affinity for air rather than for the sulphides. They form the principal part of the framework of the froth though they form only a small proportion of the oil mix. Froth flotation made the flotation art a practical success as it was accompanied by the use of small quantities of oils. The average oil use is today below one per cent of the weight of ore treated and a half per cent use would be nearer the average use than the higher figure.
In the matter of preferential treatment the flotation art has advanced very little. The oils make very little selective difference in the sulphides floated and various attempts to intensify the selective effect have not resulted in much advance. As the art stands today it is increasingly ineffective on material which is coarser than 40 mesh largely because of occluded gangue matter which weakens the flotation effect. The recovery on the very finest slime is also unsatisfactory. Any sulphide ore which has not been exposed to oxidation will float well on being ground to 40 mesh or finer and on those portions of it which will settle promptly in water.
When the process was at its high tide of enthusiasm some years ago it was predicted that it would displace all forms of sand and slime concentrating apparatus. It was even predicted that it would become the only process which need be employed even on those ores from which concentrate could be obtained after a preliminary crushing, for the process was so simple, the extraction so high, etc., that it would overcome the costs and disadvantages of fine grinding. The truth of the matter is that the flotation process is largely but an adjunct to older methods of concentration and that is but an extra string to the bow of the metallurgical engineer. If results cannot be obtained from one mode of treatment an entirely different one will often yield results. If one concentration method will not yield a satisfactory modicum of concentrate and entirely different one may yield an additional one worth while. The old guiding law of concentration to "begin it as coarse as one can" is just as true today as it ever was.